1. Field of the Invention
The present invention relates to the size detection of a document in an image reading apparatus, a digital copying machine or the like.
2. Description of the Related Art
The document size is an important quantity which relates almost all processing of the copying operation in a digital copying machine or the like, and it has to be detected correctly in order to improve the performance of a copying machine. The copying operation is performed on the basis of the detected document size.
In a digital copying machine, a document size is detected with use of sensors positioned at predetermined locations.
However, the document size detection may be erroneous when dusts or damages exist on a platen glass or on a platen cushion at the above-mentioned sensor positions.
The document size is also detected by reading image data by sampling image data signals of predetermined lines in the main scan and subscan directions. The copying operation is performed on the basis of the detected document size.
However, the document size detection may be erroneous when dusts or damages exist on a platen glass or on a platen cushion at the above-mentioned predetermined lines.
Further, when the sampling is performed with a linear image sensor for the size detection in the main scan and subscan directions, a line buffer of the capacity of a plurality of lines is needed if the real-time processing of the size detection is not performed. Then, previously, the scanner is set to be moved at a constant speed in the subscan direction for the real-time processing by using a line buffer of the capacity of a single line.